1. Field of Invention
This invention relates to glass-ceramic composite packages for integrated circuits in general, and in particular to those containing multi-layer, interconnected thick-film wiring patterns obtained by co-firing a glass-ceramic composite and copper, silver, or gold based conductors at temperatures not exceeding about 1000.degree. C.
2. Description of Prior Art
Multi-layer ceramic substrates for mounting integrated circuit chips generally comprise alternating layers of metallic circuits and ceramic insulating layers to form three dimensional interconnect circuits. The substrates are produced either by a thick film printing method or a green sheet lamination method.
The thick film printing method has been used to fabricate hybrid circuits and multi-layer printed interconnect boards. In this process, metal powders and ceramic powders are formulated into metal and dielectric (insulator) inks and then alternately screen printed onto a fired ceramic base. Generally two or three printings of dielectric material are required for every insulating layer and the circuit must be fired after each printing process. Thus, this method is very time consuming because of the large number of printing and firing steps required. The method is also prone to low production yields and is limited in the density of interconnect circuitry. Ceramic layer hermeticity is a major problem affecting yields and is a direct result of using screen printing methods to form insulating layers. In addition, conventional metal pastes contain active bonding agents to promote adhesion to ceramic substrates (e.g., lead borosilicate glass and bismuth oxide) which function acceptably in air fired applications, but which are problematic in nitrogen firing applications.
According to the green sheet lamination method, green ceramic sheets on which metal circuits have been printed are successively laminated and then co-fired to form a monolithic interconnect structure (package). Generally, the ceramic green tape is fabricated by the doctor blade casting process from a slurry containing a mixture of ceramic powders, thermoplastic resin, solvents, and other additives (dispersants, plasticizer). Polyvinyl butyral (PVB) is the most commonly used resin system for tape formation. The green tape is blanked into sheets and registration holes are punched. Via holes, which in the final package serve as vertical interconnects between layers, are punched using fixed tooling or a numerically controlled punch press. The via holes are filled and circuit trace patterns are printed using the desired metallization compositions. The individual sheets are then stacked in the proper sequence and laminated to form a solid, composite laminate. The laminate is fired to decompose and remove the organic binder and to sinter the ceramic and metal particles, thus forming a dense body containing the desired three-dimensional wiring pattern.
Aluminum oxide, because of its excellent electrical (insulating), thermal, and mechanical (especially strength) properties has been the ceramic of choice for such substrates. These ceramic bodies, generally containing 4-10 weight percent glass, require sintering temperatures above 1500.degree. C., which thus necessitates the use of refractory metals such as molybdenum or tungsten for the wiring. These metals have poor electrical conductivity as compared to highly conductive metals such as copper, and secondly, they require the use of strongly reducing atmospheres during co-firing necessitating expensive furnace systems. Alumina has been an adequate dielectric material for microelectronic packaging in the past; however, the advent of higher frequency and higher speed devices has made clear the deficiencies of the current materials systems. Al.sub.2 O.sub.3 has a relatively high dielectric constant of about 9.9, causing high signal propagation delay and low signal-to-noise ratio (crosstalk). Furthermore, alumina has a thermal expansion of 6.7.times.10.sup.-6 /.degree.C. (20.degree.-200.degree. C. range) as compared to about 3.0-3.5.times.10.sup.-6 /.degree.C. for silicon, which represents significant mismatch in thermal expansion and results in design constraints and reliability concerns (e.g., flip chip technology). Furthermore, the binders used to fabricate green tape do not decompose cleanly during firing at low temperatures (200.degree.-600.degree. C.) in the reducing atmospheres utilized; significant graphitic carbon is generated which requires a high temperature burnout treatment (1100.degree.-1200.degree. C.) prior to raising the temperature to the peak firing condition.
Accordingly, there exists a need for a materials system which allows co-sintering of the ceramic with a conductive metal such as copper, gold, or silver. An IC package fabricated from this system would have significantly improved signal transmission characteristics. To this end, a glass-ceramic material sinterable to a high density at temperatures less than 1000.degree. C. is desirable. To allow co-sintering with copper in a reducing atmosphere, in particular, the binder material must depolymerize and burnout cleanly, which precludes the use of conventional binders such as PVB. PVB or similar polymers would result in a porous ceramic and carbonaceous residue, thereby deteriorating the mechanical strength and electrical insulation. There also exists a need for a metallurgical system that yields good conductivity, adhesion and solderability when co-fired with the ceramic dielectrics. Furthermore, for optimum yields and performance, the bonding agents and ink vehicle system should be compatible with gold, silver/palladium alloys, and copper and should be free from bismuth- and/or lead-containing compounds.
There have been numerous attempts to make such a low temperature co-firable substrate; see for example: Utsume, et al., U.S. Pat. Nos. 4,536,435; Takabatake, et al., 4,593,006; Herron, et al., 4,234,367; Kamehara, et al., 4,504,339; Eustice, et al. (IEEE 36th ECC Proceedings, 1986, pp. 37-47); Nishigaki, et al., Proceedings of the 1985 International Symposium on Microelectronis (ISHM), pp. 225-234. Although some similarities exist between this art and the present invention, critical differences and improvements are realized in the present invention. With the exception of Herron, et al., who utilize a cordierite glass-ceramic, the above generally use Al.sub.2 O.sub.3 glass composites fabricated by mixing Al.sub.2 O.sub.3 powder and glass frit. These composites generally have dielectric constants between 7.0 and 8.0, higher than what would be desirable in an advanced electronic package.
A second area of importance to the co-fired package, for which the present invention provides improvements, is metallization composition and firing atmosphere. Noble metals, such as gold, silver, and silver/palladium alloys, have received primary focus in the prior art because these metals can be co-fired in an air atmosphere and the tape system can utilize state of the art binder technology. Herron, et al. and Kamehara, et al. focus on copper metallurgy and firing in a nitrogen atmosphere containing water vapor to aid in binder removal. Herron, et al. disclose a cordierite glass-ceramic with copper metallization; however, in this case the very low thermal expansion coefficient of cordierite (1-2.times.10.sup.-6 /.degree.C.) as compared to copper (17.times.10.sup.-6 /.degree.C. coupled with the low strength of their cordierite make fabrication yield and reliability problematic. Fracture in the glass-ceramic around metallization, especially vias and attached leads or pins, caused by the differential thermal expansion induced stresses during thermal cycling is particularly problematic. In addition, proper crystallization of the cordierite glass-ceramic during binder burnout and co-firing can be difficult to control. Kamehara, et al. disclose a low strength (28,000 psi), yet higher thermal expansion of their glass-ceramic (4.5.times.10.sup.-6 /.degree.C.), thus thermal expansion mismatch may not be as serious.
Kamehara et al. further disclose the addition of as much as 1% alkali oxide to the glass ceramic, which provides alkali ions that are known to enhance copper (or silver if applicable) migration in the glass during co-firing, which may lead to degradation of electrical insulating properties. Furthermore, the type of copper ink disclosed (Du Pont 9923) is known to contain bismuth oxide and/or a lead borosilicate glass, which are subject to reduction to metallic bismuth and lead during firing in the reducing atmospheres required for copper co-firing, an effect which may lead to degradation of the conductor properties. They also disclose a method of providing vias which entails filling the vias with a row of copper balls; the advantages and usefulness of this method is not made clear. However, this method seems less practical than the presently accepted screen printing method for vias using metal inks.